The invention relates to the general field of controlled cooling of hot plate or strip shaped metal and specifically to the accelerated cooling and direct quenching of steel strips and plates.
The controlled cooling of hot rolled steel is very important for achieving the desired microstructure and properties. Modern plate and hot strip mills generally use powerful cooling systems for this purpose whereby the accurate control of the temperature and the cooling rate are very important. Water is often used as a cooling fluid.
There are many different designs of cooling system available from the prior art. One of the most common types is the U-tube type laminar cooling header. The main water supply is via a large diameter pipe and the water flows out of a plurality of U-tubes and down onto the product which is being cooled. The reason that U-tubes are used is so that the main supply pipe stays full of water even when the flow is switched off. This means that the time delay between switching on the flow and water coming out of the U-tubes is minimized. It also means that when the flow is switched off only a small quantity of water drips out of the U-tubes.
However there are a number of limitations with U-tube type headers. In practice it is found that U-tubes only give a sharply defined flow pattern over a limited range of flows. The ratio between the minimum and maximum flows which give a good flow pattern is typically about 3:1. Another limitation is that the jets are a large distance above the product which is being cooled which reduces the cooling efficiency.
Due to the limitations of conventional U-tube designs, many modern systems use multi-jet type headers instead. Some of these designs are described in EP 0 176 494, EP 0 178 281, EP 0 233 854 and EP 0 297 077. A main water supply pipe feeds water into a header. Inside the header are a large number of nozzles which produce a large number of water jets. There are a number of advantages to this type of multi-jet header design. The large numbers of jets provide much greater cooling power than U-tube type headers. In addition the design allows the jets to be much closer to the product being cooled and this further increases the cooling power. The large numbers of small jets also allow a much wider range of stable flows to be used. The ratio between the minimum and maximum stable flows is 20:1 or more compared to around 3:1 for U-tubes.
Whilst the multi-jet type header offers many advantages over the U-tube type headers it does have some disadvantages. When the flow is switched off the water in the supply pipe drains out through the nozzles. This is undesirable because the water could drip onto products that do not require any further cooling. It also means that when the flow is switched on for the next product that does require cooling the supply pipe has to be re-filled before the flow is properly established.
Another undesirable feature is that at low flows it takes a long time to change the flow. The reason for this is that the flow out of the nozzles is proportional to the square-root of the pressure at the nozzles. At maximum flow the pressure in the header is typically about 4 bar or roughly 40 meters head of water. With a 20:1 ratio between minimum and maximum flow, the pressure required for minimum flow is therefore only 40/(20×20) meters which is only 0.1 meters. Since the supply pipe is typically 300 mm in diameter this means that for minimum flow the supply pipe is only partially full. If the flow into the supply pipe is changed the flow out of the nozzles will not match the flow into the supply pipe until the water level in the pipe has reached the correct new equilibrium level. This can take up to 100 seconds or more at very low flows.